“but the hills too will be yours. Although it is a forest, you will clear it, and the resulting open land will be yours. You will drive out the Kena‘ani, even though they have iron chariots and are strong.”
You have to take ownership of your possession. It lays before you and you can take it. You will have to work for it, but it is yours, if only you would boldly seize it.
The verses before this speak of the Josephites complaining that their portion isn’t enough because they are too numerous. The verses also say that they had not expelled all the Canaanites from their land either. Joshua pushes back and tells them that they have the might to do it and beyond that they have the ability to expand into the forest land around them to make use of it. They have the ability, but they lack the will. Perhaps a better way to say it, is that they have the will but they refuse to exercise it. That is such a universal problem. We often shrink back from achieving something for myriad excuses, but the truth is, if we want something we can make it happen. The reason the Josephites came and complained to Joshua is because they lacked sufficient motivation. Perhaps they lacked faith, I can’t really say. I am reading in enough, I don’t want to push this passage too far with my own interpretation or agenda. When Joshua assured them they could overtake the people and the land, the Josephites were left with a choice: stay unsatisfied or take action and improve their lot.